Electrically conductive sheet materials, for example electrically conductive textile sheet materials, are of interest for a multiplicity of applications. A key application is the use of electrically conductive sheet materials in an energy storage medium, particularly for storing renewable energies, for example hydrodynamic power, wind energy, solar radiation and geothermal energy, and for storing energies generated from renewable raw materials.
A wide variety of batteries are often used for energy storage. These batteries are employed in the form of primary or secondary cells, or in electroplating, NaCl electrolysis and the electrolytic synthesis of inorganic compounds. Nonrechargeable batteries are known as primary cells. Secondary cells, which were developed to store surplus electrical energy, have moved into the focus of research and political attention in view of the scarcity of fossil raw materials and the use of renewable energies. Nickel-cadmium, lead-acid, lithium-ion and nickel-metal hydride batteries are the best-known secondary cells.
A redox flow cell is a type of rechargeable battery where electrical energy is stored in chemical reactants dissolved in a solvent. In effect, two energy-storing solutions of electrolyte circulate in two separate circuits wherebetween ion exchange takes place in the cell, by means of a membrane. The energy-storing solutions of electrolyte are stored in separate tanks outside the cell, so energy quantity as well as power output can be scaled independently of each other. The tanks are simple to fill manually. Because the solutions of electrolyte are exchanged, they can also be charged and discharged in separate batteries. Cell voltage is typically between 1.0 and 2.2 V. This holds for vanadium redox flow batteries in particular.
A membrane divides the redox flow cell into two half-cells. The membrane is permeable for the counter-ions of the electrolyte solutions and each half-cell is bounded by an electrode where a chemical reaction—reduction or oxidation—takes place.
The membranes used are typically microporous separators, which allow all ions to pass through, or selective anion or cation exchanger membranes. The membrane is designed to prevent the two solutions of electrolyte from mixing.
The electrodes, which are designed to be useful for a high range of electrochemical voltage in aqueous solutions, usually consist of graphite, in general. Graphite felts having a high specific surface area are used as electrode material for a very high specific power output.
JP 4632043 B2 discloses a carbon fiber felt obtained by carbonization of oxidized polyacrylonitrile (PAN-OX) fibers, while a felt is obtainable by needlepunching. Owing to the fiber structure of the felt described in JP 4632043 B2, fiber alignment and conductance alignment have mutually different orientations. As a result, the current has to pass through a multiplicity of fibrous nodes, which greatly increases the electrical resistance of the felt.
JP 2003 308851 A describes an electrode material comprising a nonwoven fabric comprising carbonaceous fibers. It is formed at high temperatures under an inert atmosphere. The felt structure further limits the porosity and thus also the pressure drop. JP 2001 085028 A shows a carbon electrode material for use in a redox flow battery in aqueous systems of electrolyte.
The electrode materials described in JP 2003 308851 A and JP 2001 085028 A do not have directed structures, which makes it difficult for a fluid, in particular an electrolyte solution, to flow therethrough. This leads to increased electrical resistance and a high pressure drop during the flow through the electrode material. The aforementioned electrode materials further have a rough surface which, on contact with a bipolar plate, increases the electrical resistance. A high degree of fiber irregularity on the part of these materials amplifies abrasion to shorten the useful life of these electrode materials.
DE 100 505 12 A1 describes a conductive nonwoven fabric comprising carbon fibers. This nonwoven fabric is from 80 to 500 μm in thickness.